A compression-type refrigerator is generally comprised of at least a compressor, a condenser, an expansion mechanism (such as an expansion valve) and an evaporator or comprised additionally of a dryer, and is constructed such that a mixed liquid composed of a refrigerant and a lubricating oil (refrigerator oil) is circulated through a closed system including these devices. In such a compression-type refrigerator, in general, a high temperature is established within a compressor while a low temperature is established within a cooler, though depending upon the devices used. Therefore, it is necessary that the refrigerant and the lubricating oil can be circulated through the system without causing a phase separation in a wide temperature range encompassing from the low temperature to the high temperature. In general, the refrigerant and lubricating oil have temperature regions on a low temperature side and on a high temperature side in which regions they cause phase separation. The highest temperature in the low temperature side separation region is preferably −10° C. or lower, particularly preferably −20° C. or lower. On the other hand, the lowest temperature in the high temperature side separation region is preferably 30° C. or higher, particularly preferably 40° C. or higher. Phase separation during the operation of the refrigerator considerably adversely affects the service life and the operation efficiency of the apparatus. For example, if phase separation between the refrigerant and lubricating oil occurs in the compressor section, lubricity in a moving part will become insufficient so that seizing and other problems will occur to considerably reduce the service life of the apparatus. On the other hand, if phase separation occurs in the evaporator, the heat exchange efficiency is considerably reduced because of the presence of a highly viscous lubricating oil.
As a refrigerant for refrigerators, a chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC), etc. have been hitherto mainly used. Because these compounds contain chlorine atoms which cause environmental problems, chlorine-free refrigerants such as a hydrofluorocarbon (HFC) have been investigated. Thus, hydrofluorocarbons which are saturated fluorinated hydrocarbon compounds, typically 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, 1,1,1-trifluoroethane (which are referred to as R134a, R32, R125 and R143a, respectively) have become a focus of attention. For example, R134a is actually used in a car air conditioning system. Further, as an HFC refrigerant that has a lower global warming coefficient as compared with the above HFCs and that can be employed in the current car air conditioning system, there has been found a hydrofluorocarbon refrigerant (see, for example, Patent Document 1) which is an unsaturated fluorinated hydrocarbon.
As a main lubricating oil for use in a refrigerator using an HFC refrigerant, polyvinyl ether (PVE) and a polyalkylene glycol (PAG) are known (see, for example, Patent Document 2 and Patent Document 3). There is, however, room for further improving these lubricating oils with respect to compatibility with the refrigerant.    [Patent Document 1] Japanese Translation of PCT International Application Publication No. 2006-503961    [Patent Document 2] Japanese Unexamined Patent Application Publication No. H06-128578    [Patent Document 3] Japanese Unexamined Patent Application Publication No. H02-305893